Fading is a common problem in two-way radio communications. Two types of fading affect mobile communications channels, namely, large scale and small scale fading. Large scale fading represents the average signal power attenuation due to travel over a large area and is affected by terrain, buildings, and other obstacles. Small scale fading manifests itself as time spreading of the signal (signal dispersion) and time variant performance of the channel. For mobile radio applications, the channel is time variant due to the changing of position between the receiver and transmitter. Frequency selective fading arises due to time-dispersion in the multi-path channel, such as a wireless cellular channel. This type of fading causes irreducible errors unless the effects of frequency selective fading can be minimized.
Reliable signal detection in a frequency selective fading channel becomes difficult as channel filtering distorts the transmitted waveform. Strong coding techniques, like turbo codes and soft decision viterbi decoding, mitigate the affects of fading and improves decoding at low signal strengths. As a result, the limiting factor in error rate performance is the ability to detect the presence of the received signal in fading. Missed detections lead to irreducible error rate floors and a decrease in throughput.
An optimal signal detection method utilizes a matched filter since this type of filter works to maximize received signal-to-noise ratio. In the prior art, the use of a matched filter to search for a known time domain waveform is a well-known signal detection method. A detection is determined if the magnitude squared of the matched filter output rises above a predetermined threshold. This scheme generally results in good detection performance in an Additive White Gaussian Noise (AWGN) channel. However, in a channel exhibiting frequency selective fading, the channel can distort several frequency components of the received signal. This results in low matched filter outputs, thus degrading detection performance and leading to a greater likelihood of missed detections.